The present invention relates to a method an apparatus for improving the efficiency of an acousto-optic modulator or deflector by successively returning the undiffracted beam through the acoustic wave within the modulator. The novel part is the method of keeping the successive beams co-axial.
Acousto-optic modulators and deflectors operate in the well known Bragg mode. An acoustic wave is set up in a crystal and a beam, typically produced by a laser, enters the acoustic wave at an angle (known as the Bragg angle) to maximize diffraction efficiency. Part of the beam is diffracted and part continues straight through the acoustic wave. For low efficiency devices most of the light is undiffracted and therefore wasted (for those applications were only the diffracted beam can be used). When the frequency of the acoustic wave is constant the device is usually referred to as a modulator and if the frequency is variable, causing a change in the diffraction angle, the device is referred to as a deflector. The current invention is suitable for both applications.
In order to re-use the wasted undiffracted light many schemes were proposed, however they all suffer from one common disadvantage. All prior art re-uses the undiffracted light by returning it to the modulator in a serpentine or spiral path. This was required since the mirror returning the light should not block the incoming beam, thus the incoming beam and the returned beam can not be co-axial. The disadvantage of these methods is that the successive beams take up a wider area of the acoustic wave than a single beam. The bandwidth of an acousto-optic modulator is limited by the acoustic transit time, thus using a wider light beam reduces the bandwidth of the device. An example of prior art is U.S. Pat. No. 4,473,275 using successive passes in a spiral manner. Another example is U.S. Pat. Nos. 3,988,055 using successive passes in a helical manner. U.S. Pat. Nos. 3,492,063 and 3,923,380 use successive passes in a serpentine fashion. No prior art teaches successive passes without causing the beam in the acousto-optic modulator to widen with each pass, therefore in all prior art there is a speed or bandwidth penalty paid for increasing the efficiency.